The present invention relates generally to force sensitive transducers and, in particular, to an apparatus for measuring the work forces present in a machine tool.
The ability to measure force or load is a vital part of many industrial processes. Recognizing that a structure undergoes dimensional changes when subjected to a load, the measure of the dimensional change in a structure can accurately indicate the amount of force applied thereto. Such measurement is made through the use of a wire, foil, or semi-conductor element intimately bonded to the structure. By measuring the change in the electrical resistance of the element, which resistance is a function of the change in cross section of the element due to alterations in the dimensions of a structure, it is possible to accurately measure the load applied to the structure.
Many transducer devices have been developed for attachment to structures for measuring the amount of stress applied thereto. U.S. Pat. No. 3,444,390 and U.S. Pat. No. 4,171,646 both disclose measuring devices for use on machine tools such as forges, presses and the like. Generally, prior art devices are not sufficiently sensitive or physically constructed to be readily adaptable for use in working areas of a machine near the tool in order to directly measure forces on the tool. For example, the transducer arrangements disclosed in the above-mentioned patents are incapable of directly measuring the working load on a tool. This inability to directly measure tool loading is a particularly acute problem when it is desired to monitor the individual loads on a plurality of tools carried on a common base and caused to do work simultaneously.
Ideally, the apparatus for measuring the work forces in a machine tool is mounted in an area of the machine which is shielded to protect the apparatus from physical damage resulting from inadvertent impact or from contamination by debris or lubricants. It also desirable that the load measuring apparatus be readily accessible for inspection, removable, and repair or maintenance.
U.S. Pat. No. 4,412,456 and U.S. Pat. No. 4,526,044 both disclose load sensing probes for disposition in a cavity in a back-up member arranged to support a tool at a working station for measuring the strain associated with working forces applied to the tool. Each probe has an elongate configuration which allows it to be mounted at a point remote from its load sensing end and enables it to be conveniently used with a plurality of similar probes in relatively confined spaces. A split plug includes a slot defining two sensing arms, which arms are pushed outwardly into engagement with the inner wall of an accommodating cavity formed in a back-up member of the machine tool. The probe is sensitive to relative closing movement of the cavity caused by compressive strain in the back-up member.
Each of the probes includes a split plug portion having an enlarged head, a cylindrical shoulder, an elongate stem, and a disc-shaped sensing member. A plurality of conventional strain gauges are mounted on the sensing member to detect compressive strain. A central aperture extends through the longitudinal axis of the split plug. A slot is formed in the sensing member and a portion of the stem defines two moveable sensing arms. A split plug pin having an outwardly tapered end portion is longitudinally inserted through the aperture into threaded cooperation with a nut. Upon tightening the nut, the threaded end of the pin is drawn upwardly within the split plug, causing the sensing arms to be pushed outwardly into frictional engagement with the inside wall of the accompanying cavity until a desired predetermined level of preloading is reached. Tool working forces result in relative closing movement of the cavity, which movement causes dimensional changes in the sensing member which can be measured by the strain gauges.
Each of the above-described probes is selective in its sensitivity so that it can be employed in a face plate at a discreet point in a limited area as compared to the total projected area of the face plate. Accordingly, several similar probes, one for each tool station, can be utilized in a single face plate supporting a plurality of spaced tools.